The present invention relates to flowmeters such as vortex shedding meters or swirlmeters which are responsive to a fluid flow.
Flowmeters sense the flow of liquids or gasses in conduits and produce a signal indicative of the flow. Under certain circumstances, the presence of an obstacle known alternatively as a shedding bar, bluff body, or vortex generator, in a flow conduit causes periodic vortices in the flow. The frequency of these vortices is directly proportional to the flow velocity in the flowmeter. The shedding vortices produce an alternating differential pressure across the bluff body at the shedding frequency. This differential pressure is converted to an electrical signal by piezoelectric crystals or other differential pressure devices. The magnitude of the differential pressure or electric signal is proportional to xcfx81V2, where xcfx81 is the fluid density and V is the fluid velocity. When the ratio of pipe diameter to the size of the bluff body is held constant, the signal magnitude is proportional to xcfx81D2F2, where D is the inside diameter of the metering pipe and F is the shedding frequency. The vortex flowmeter produces pulses having a frequency proportional to the flow rate. In a swirlmeter, the fluid whose flow rate is to be measured is forced to assume a swirl component by means of swirl blades, the arrangement being such that the swirling motion is transformed into precessional movement to produce fluidic pulses which are sensed to yield a signal whose frequency is proportional to flow rate. See e.g., U.S. Pat. Nos. 3,616,693 and 3,719,080 which disclose examples of swirlmeters and are hereby incorporated by reference.As used herein, xe2x80x9cvortex flowmeterxe2x80x9d shall include both vortex shedding meters and swirlmeters.
The vortex flowmeter is a measurement transmitter that is typically mounted in the field of a process control industry installation where power consumption is a concern. The vortex flowmeter can provide a current output representative of the flow rate, where the magnitude of current varies between 4-20 mA on a current loop. It is also desirable for the vortex flowmeter to be powered completely from the current loop so that additional power sources need not be used. Thus, the vortex flowmeter measurement transmitter should be able to operate with less than 4 mA in order for the transmitter to adhere to this process control industry communication standard.
It is known to incorporate a microprocessor into a vortex flowmeter. The microprocessor receives digital representations of the output signal from the vortex sensor and computes desired output quantities based on parameters of the digital representation. For instance, a vortex flowmeter can calculate the mass flow rate through the pipe or conduit. It is desirable to provide the calculated mass flow rate approximately ten times per second. For each new calculation of the mass flow rate, the microprocessor must perform many mathematical steps wherein each mathematical step requires a number of clock cycles, thus limiting the rate at which calculated mass flow rates can be provided. Although it would be desirable to use a more powerful microprocessor, which could perform further calculations to improve accuracy, the microprocessor would require more power than is available from the 4-20 mA industry standard discussed above.
Nevertheless, there is a continuing need for a vortex flowmeter having improved accuracy. However, sacrifices should not be made in the update rate nor should power consumption exceed the power available from the current loop.
A method for measuring fluid flow includes generating vortices in a fluid and relating the fluid flow to a first set of fluid parameters to obtain a first relationship, and relating the fluid flow to a second set of fluid parameters to obtain a second relationship. The first and second sets of fluid parameters are monitored. A first flow value is calculated from the first set of monitor fluid parameters and the first relationship. The second relationship is adjusted based on the first flow value. The output value is calculated from the second set of monitored fluid parameters and the adjusted second relationship.